This invention relates to a hybrid ship hull in which different sections are made of different materials, and more particularly to a hybrid ship hull whose stern and bow section are made of composite materials while its mid-section is made of a hybrid steel framing with a composite skin, of advanced double-steel hull or of conventional steel hull, especially for use with ships of lengths of at least about 300 feet or greater.
Current ship hulls are normally made of steel which is magnetic and thus entails the well-known disadvantages thereof, especially during war-time conditions. Traditional shipyard designs use conventional single-hull constructions with longitudinal stringers and transverse framing. To achieve non-magnetic capabilities, stainless steel hulls are recently being investigated for the next generation of Navy ships.
Current Navy ships have simple bow and stern geometries. Advanced hulls, such as the tumblehome hull envisioned by the Navy, may use water jet propulsion systems, a modified water jet, shrouded propellers or other complex geometry. The complex stern section, associated with these propulsor systems, could be long sections and require double curvature and appendages, which are expensive to form using steel plating or forging. In addition, steel construction would make the bow and stern sections extremely heavy.
The revolutionary xe2x80x9cwave piercingxe2x80x9d bow of advanced hull forms, such as the tumblehome hull, is envisioned to have also complex curvature for stealth, seakeeping or maneuvering, not seen in any previous ship construction. Forming steel for a long bow section with double curvature would be very expensive and extremely heavy. The heavy mass concentration of a steel stern and bow would create problems in maneuvering and seakeeping. Current steel construction of ships would give a very heavy bow and stern section, which leads to large whipping moments in underwater explosion.
The manufacturing process of steel hulls involves welding which, in turn, produces residual stresses leading to large plate (dishing) deformations, which are also called at times xe2x80x9chungry horse.xe2x80x9d These deformations reduce the fatigue life and stealth characteristics of the hull. To assure manufacturing tolerances, it is necessary to relieve the residual stresses by heat treatment, which is a very high cost operation, or to use some advanced welding technology that would minimize residual stresses, such as, for example, laser welding, which, however, is generally not yet available at shipyards. To date, the alternative is to build the hull out of composite materials, known as such in the art. However, several studies have indicated that for hulls longer than about 200 feet, even carbon fiber composites do not provide the necessary stiffness and strength required for the hull. Furthermore, the cost of carbon fiber composites which is currently $12-18 per pound of carbon fiber as compared to $0.45xcx9c$0.5 per pound for high strength steel, would be prohibitive for ships of this size. Low cost, high performance composite materials such as glass fiber composites (GRP) using resin transfer molding processes which are presently used in patrol boats, Corvettes and mine hunters, do not offer the stiffness nor the in-plane strength required for long hulls of combatant ships or other large commercial ships. The load-carrying mechanism for long ships is by axial tension and compression in the hogging and sagging mode between waves. The technology of composite sandwich construction, which is common in smaller ship lengths or boats, does not satisfy the carrying capability for sea loads in longer ship hulls. The in-plane strength of the composite material is therefore critical. Moreover, for small ships and boats, the bending strength of the composite material is critical. The present technology of composite sandwich construction which is common in smaller ship lengths or in boats would not add to the carrying capability for sea loads in long ship hulls.
Advanced double-hull constructions of the type disclosed in U.S. Pat. Nos. 5,218,919 and 5,477,797 are presently under development. Additionally, for naval applications, a modification of these concepts, as disclosed in U.S. Pat. No. 5,582,124 is presently considered. Composite hulls for naval vessels is also presently being considered. Composite hulls for naval vessels of lengths less than 300 feet are presently built with the use of GRP or carbon fiber sandwich construction using a patented process called xe2x80x9cSCRIMPxe2x80x9d (U.S. Pat. Nos. 4,902,215 and 5,958,325). In these types of constructions, the entire hull is made of the same material which is different from and must be distinguished from a hybrid construction according to this invention where more than one material is used.
The U. S. Pat. No. 4,365,580 to Blount discloses a steel hull construction forming an inner box-like structure with a fiberglass outer hull. In this patent the steel box thereby carries all the sea loads such as bending moments and shear stresses while the composite shell and foam transmits the water pressure to the box. The construction according to this patent therefore resembles a steel hull covered with an add-on parasitic composite skin that gives it the shape. The other patents mentioned in the Blount ""580 Patent are sandwich-type constructions in which a synthetic foam material is sandwiched between inner and outer shells and hence are not hybrids of two different materials. Additionally, in the construction according to the Blount ""580 Patent, the composite material is an added weight to the steel load-carrying hull which is detrimental to speed and efficiency. Furthermore, in the ""580 Patent, the bow and stern have no load-carrying capability and would not work for a naval combatant ship or larger commercial vessel as contemplated by the present invention.
U.S. Pat. No. 5,778,813 to Kennedy discloses a composite laminated panel for containment vessels such as double-hull oil tankers. It is composite in the sense that it involves a steel double hull with an elastomer core in between. However, this patent also does not disclose or suggest the present invention because the steel thereof carries all sea loads, and the elastomer merely acts in shielding the inner hull from cracks when the outer hull is pierced, ruptured or penetrated.
The present invention is based on the concept to subdivide the hull of a ship into three parts, i.e., the bow section, the middle section and the stern section which utilize different materials for their construction. The bow and stern sections are thereby made of a composite material such as glassed reinforced plastic (GRP) material while the mid-section utilizes a steel framing. In one embodiment, the mid-section is made of a hybrid stainless steel framing with a wetted outside skin or, alternatively, of an outer skin and of an inner shell for a double hull whereby the composite sections may have a length of about 20 feet to about 30 feet. In another embodiment of this invention, the mid-section of the hull would be made of Stainless Steel Advanced Double Hull construction (SSADH) as disclosed, for example, in U.S. Pat. No. 5,582,124. The hybrid stainless steel arrangement has a novel arrangement of longitudinal beams joined by vertical and horizontal beams is provided whereby at least some of the beams are made of stainless steel and preferably are box-type beams. The two truss-like structures on the port and starboard sides are connected by top and bottom horizontal beams, thereby forming a box-like structure to assure the required lateral and torsional stiffness and to resist beam and oblique sea loads. Furthermore, to connect the outer skin panels to the steel frame of the mid-section of the first embodiment, shear connectors are preferably provided along the entire length of the longitudinal box beams as well as the vertical and horizontal framing which utilize a novel construction of two concentric cylinders. To provide dynamic load attenuation, an elastic material may be sandwiched between the composite and the framing. To connect the bow and stern sections with the mid-section, a novel arrangement of pre-stressing cables are used between the bow and the mid-section as also between the stern and the mid-section whereby these pre-stressing cables have a moment-carrying capacity equal to the total moment-carrying capacity of the composite bow and stern cross section. Additionally, water-tight end bulkheads are preferably provided at the transitions from the mid-section to the bow and the stern sections. To eliminate peeling or de-lamination, a lap-type connection between the composites of the end sections and the steel framing may be used by staggering the skin as well as a terminal plate-like part of the steel mid-section, and/or stiffness thereof. Punched holes are provided in the steel plate and/or stiffeners for stitching out-of-plane glass or carbon fibers, whereby the stitching operation precedes the co-curing process of the composites. In the alternative, a scarf joint may be used in the connection between the steel plate and/or stiffeners and the composite panels of the end sections.
Two types of construction are therefore proposed for the main or mid-section according to this invention. One type of construction utilizes a stainless steel framing with a wetted outside skirt or, alternatively, an outer skin and inner shell for double-hull construction, whereby the outer sections are preferably made of composite sections with a length of about twenty to about thirty feet. The alternative construction for the mid-section of the hull according to this invention utilizes only stainless steel and is preferably of Stainless Steel Advanced Double-Hull construction (SSADH) as disclosed, for example, in U.S. Pat. No. 5,582,124.
The three-section ship hull construction of this invention provides a stealthy, affordable ship hull whose hybrid hull with its composite bow and stern section would allow the manufacture of any shape necessary to meet signature requirements at much lower cost. Furthermore, the light-weight stern and bow sections would lead to superior maneuvering and sea-keeping, maneuvering, fuel efficiency and speed, in addition to reducing the whipping moments in case of underwater explosions.